Common conventional image forming apparatuses employing an electrophotographic process, such as laser printers, copiers, and facsimile machines, form electrostatic latent images on a photosensitive member by charging the surface of the photosensitive member with a Scorotron charger or other charging device and exposing the charged surface of the photosensitive member to a scanned laser beam based on predetermined image data. Next, the image forming apparatus develops the electrostatic latent image with a developer unit, including a developer cartridge or the like for depositing toner or other developer on the surface of the photosensitive member, and transfers the developed image onto a recording medium, such as paper, with a transfer roller or other transfer unit to form an image on the recording medium.
The processes described above require a power device for supplying power of a predetermined high voltage to each of the charging device, developing unit, transfer unit, and the like,
Japanese Patent Application Publication No. 10-91258 discloses one conventional method employed in a high-voltage power device for achieving the above-mentioned power supply. This method involves outputting a pulse width modulation (PWM) signal to a switch element connected in series to the primary side of a transformer, feeding back the output voltage produced on the secondary side of the transformer, performing an arithmetic operation on the output voltage by making use of the linearity of the duty-output characteristics of the transformer, and modifying the duty of the PWM signal to achieve the target output voltage. This method can produce an output voltage over a wide range using a single transformer.